About the NZ team - Astle is completely safe I think. Everyone seems to say he hasn't scored anything since his return, but that's not completely true. He has averaged 29.21 since then which is not good by any stretch of the imagination, but its better than most of our batsmen.
Since 1st Test v England: 64, 49, 2, 8, 15, 0, 11, 39, 19, 17, 52, 38, 74, 21.
His last 4 scores there (all against Aus) show hes coming into some form.
I'm more concerned about McCullum. He seems to have made a whole heap of technical changes for some reason (e.g. opening his stance more, and shuffling down the pitch), and he hasnt scored any runs since that hundred. I think the main problem is in his head though. He doesnt seem to have any motivation to stay at the crease and grind it out - look at how many 20s hes got this tour. Part of the problem may be that there is no competition for his spot. Against SA and England he had to proove that his elavation over Hart was the right decision and as a result scored some good runs. Since then he knows no-one is close to him and seems to have slacked off a little.